ERROR: plpgsql: cache lookup for proc 17911 failed

I created a function with iscachable,

' LANGUAGE 'plpgsql' WITH (iscachable);

I then call the function.
Then I decide to change the return type of the function so I first drop
the
function, then change the return type.
The next time I attempt to execute the function I get:

ERROR:  plpgsql: cache lookup for proc 17911 failed

I would have thought that dropping a function would have also cleared it
from the cache.

Richard